The invention relates to shrouded aerofoils. More particularly, the invention concerns an assembly or sub-assembly consisting of a plurality of shrouded turbine blades sharing a common tip shroud.
Gas turbine engines frequently employ tip shrouds on individual aerofoils to limit blade amplitudes when vibrating in a random manner and to guide fluid flow over the aerofoils. Neighbouring shrouds abut in the circumferential direction to add mechanical stiffness. When a series of such assemblies are mounted together the shrouds define in effect a continuous annular surface. Opposite edges of the shrouds in the circumferential direction are provided with abutment faces and are designed to introduce to the assembly desired constraints. In order to keep natural blade frequencies high and to avoid low engine order bladed disk resonances as well as damping random blade resonances it is known to incorporate Z-notches in the abutments. These separate that portion of the shroud that retains clearance with its neighbour from that part that is abutting the shroud abutment faces. By pre-twisting the blade aerofoils, portions of adjacent shroud abutment face are maintained in frictional contact thereby constraining the assembly from certain modes of vibration.
It is also known to weld the blade tip shrouds of neighbouring blades into pairs either to raise a blade resonant frequency out of the engine operating range. Also, this can be effective to prevent shroud tilting.
The present invention according to its broadest aspect provides a shrouded aerofoil assembly comprising at least two blades sharing a common circumferential shroud ring segment which has at opposite ends in the circumferential direction notched abutment faces.